feat: past tracks layer toggle + persist vessel/crew registry

Layers menu:
- Add "Past tracks" switch to layer picker sheet — hides/shows previous
  voyage polylines on the map, persisted across sessions via SharedPrefs

Vessel/crew registry:
- VesselRepository now takes a Context and persists all vessels and crew
  to SharedPreferences as JSON (Moshi, same pattern as BoatProfileRepository)
- Wire VesselRepository as a singleton in NavApplication so the Fragment
  and ViewModel share one instance — previously each created its own empty repo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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